OXYGEN ISOTOPE EVIDENCE FOR THE ORIGIN OF GARNET IN THE PERALUMINOUS SOUTH MOUNTAIN BATHOLITH, NOVA SCOTIA

The peraluminous South Mountain Batholith (SMB), Nova Scotia, records evidence for the assimilation of wall rocks in the form of xenoliths, xenocrysts, and former wall-rock derived partial melt. Garnet (Grt) in the SMB is most commonly associated with zones of wall rock assimilation, where textural characteristics suggest that Grt occurs as metamorphic xenocrysts, or as peritectic crystals formed in xenoliths entrained in SMB magma. Away from external contacts of the batholith and away from zones obvious wall rock contamination, Grt also occurs in the SMB, where texturally the grains appear to be of metamorphic xenocrystic, peritectic, as well as primary magmatic origin. Typically, Grt is almandine-spessartine rich with X_Mg <0.3 and Mn between 0.2 and 1.2 apfu; however, major- and trace-element compositions alone do not unequivocally discriminate between Grt grains of various origins.

We present the first oxygen isotope analyses of Grt from the SMB, with the aim to evaluate the texturally defined Grt groups, and to better constrain the conditions of Grt nucleation and growth. Because Grt has slow intracrystalline oxygen diffusion, d¹⁸O(Grt) should reflect conditions at the time of crystallization.

Our analyses show the highest d¹⁸O values for Grt from the wall rocks (9.35±0.01; 3 rocks), intermediate d¹⁸O values for texturally determined peritectic Grt from partially assimilated wall rocks (8.62±0.21; 3 rocks), and the lowest values of d¹⁸O in texturally determined magmatic Grt (8.15±0.21; 6 rocks). Furthermore, values of d¹⁸O of zircon in the batholith (8.09±0.31; 3 rocks) overlap with d¹⁸O of magmatic Grt, indicating high-temperature equilibrium between the two minerals. The intermediate d¹⁸O of peritectic Grt indicates sub-equal oxygen mass contributions (~50% each) from metamorphic xenoliths and magma in which Grt nucleated. We predict that peritectic Grt is zoned in d¹⁸O; cores of Grt should be higher in d¹⁸O, owing to greater mass from the xenolithic protoliths, but progressive disaggregation and "opening" of xenoliths to magma influx should result in rims with lower, magmatic d¹⁸O values. Together, textural and d¹⁸O data are powerful tools to delineate Grt populations in peraluminous granites and to provide useful constraints on assimilation and contamination processes.